1. Field of the Invention
The present invention relates generally to a laser diode array and, more particularly, to an array including a substrate, a laser active layer, and a cover layer forming at least two ridges over the active layer.
2. Description of the Related Art
The operating principle of a laser is based essentially upon the generation of radiation in an active region which is intensified only for certain frequencies due to resonance. Laser-active regions, which are commonly formed as strip-like regions, have end surfaces which act as partly reflecting mirrors. Resonance occurs when the longitudinal extent of the active region corresponds to a whole plurality of half wavelengths of laser energy. A single frequency laser diode, for example, can be manufactured by replacing a single laser-active region by a plurality of laser active regions that are coupled, or optically joined, to one another.
In index-guided lasers, the localization of the radiant energy field within the laser-active stripes is effected by a change in the refractive index of the surrounding material. In these index-guided lasers, the intensity of the radiation field decays very quickly outside of the active layer. With a suitable layer structure in this type of semiconductor laser, lateral emission losses can be kept very low such as for high efficiency. However, lateral optical coupling of various laser-active strips of this type is difficult.
In German Patent Application No. P37 06 477.0 is disclosed a semiconductor laser arrangement comprising an index-guidance means or quasi-index-guidance means wherein lateral coupling is accomplished by elimination of the index guidance means in sections. Such complete but only sectional cancellation of the electromagnetic isolation, or guidance, of the laser active stripes from one another has a disadvantage in that the structure of the various laser resonators is not sufficiently uniform.
In German Patent Application No. P37 08 666.9 is disclosed a laser arrangement having separately drivable, coupled semiconductor lasers. Index guidance for the radiation field is also disclosed. The coupling, or optical joining, is present in a vertical direction so that the index guidance of the surrounding semiconductor material provides a relatively high degree of localization of the radiation within the laser active strips. With the arrangement disclosed therein, the vertical layer structure is considerably more involved than for laterally coupled semiconductor arrays.